High temperature niobium base alloy



United States Patent Oiiice 3,046,109 Patented July 24, 1962 3,046,109 HIGH TEMPERATURE NIOBIUM BASE ALLOY Neil M. Lottridge, Jr., Warren, Mich., assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware No Drawing. Filed May 1, 1959, Ser. No. 810,499 4 Claims. (Cl. 75-174) This invention relates to a niobium base alloy having an outstanding combination of high hot strength, fabricability and oxidation resistance at elevated temperatures. It pertains particularly to a refractory metal alloy of this type which is designed for buckets and guide vanes of gas turbine engines in which metal temperatures reach 2000" F.

The nickel base alloy and cobalt base alloy blades commonly used today in gas turbine engines for aircraft normally have maximum service temperatures of approximately 1800 F. to 1900 F. This limitation necessarily restricts the performance and efiiciency of these engines. Refractory metals, such as niobium, tungsten, molybdenum and chromium, have satisfactory high melting temperatures and sufficient potential availability to warrant investigation as high temperature turbine blade materials. However, each of these metals exhibits poor oxidation resistance at temperatures of 2000 F. or above. Such metals are therefore unsatisfactory for use in turbine blades which necessarily are exposed to extremely hot oxidizing gases. During recent years attempts have been made to correct this deficiency by adding small amounts of various alloying elements to these refractory base metals. However, these attempts have been unsuccessful since the resultant products still did not possess adequate oxidation resistance at the very high temperatures under consideration.

Consequently, a principal object of the present invention is to provide a refractory alloy which can be employed as a turbine blade material at temperatures up to 2000 F. because of its oxidation resistance at such temperatures, coupled with good hot strength and other necessary physical properties. It is considered desirable that an alloy to be used for gas turbine service in air at 2000 F. have a 100-hour stress-rupture life at that temperature under a load of 15,000 p.s.i. Such an alloy also must possess adequate fabricability and a melting point of at least 3000 F. Of course, turbine blades formed of this alloy should have adequate room temperature ductility as Well as good oxidation resistance in air at a temperature of 2000 F.

In accordance with this invention, I have found that the foregoing requirements are satisfied to an outstanding degree by a refractory alloy comprising about 2% to 5% titanium, 9% to 11% molybdenum, 4% to 8% chromium and the balance (76% to 84%) substantially all niobium. Approximately 0.5% to 2% zirconium is preferably also included in the alloy and serves to increase its strength to an appreciable extent without adversely affecting its extreme high temperature oxidation resistance. An alloy composed of about 82% niobium, 10% molybdenum, 2% titanium, 4% chromium and 2% zirconium, for example, had a total surface metal loss of less than 5 mils in thickness after 100 hours cyclic exposure in air at a temperature of 2000 F. Small quantities of various other elements, usually less than 1%, can be tolerated in the alloy without detracting from its mechanical properties.

The molybdenum serves to materially increase the oxidation resistance of the niobium base alloy and also contributes to its hot strength. When the molybdenum content is lower than about 9% or higher than approximately 11%, the oxidation resistance of the alloy is noticeably reduced.

Titanium also improves oxidation resistance, although it is also necessary in order to provide the niobium base alloy with the desired amount of ductility. If the amount of titanium present is less than about 2%, embrittlement of the alloy results. A titanium content of 5% produces greater oxidation resistance than 2% titanium, but when more than 5% titanium is present the hot strength of the alloy is reduced to an excessive extent.

Likewise, chromium contributes to the oxidation resistance of the niobium base alloy as well as increasing its hot strength. A chromium content of at least 4% is necessary for outstanding oxidation resistance, but if more than approximately 8% chromium is included the alloy becomes excessively brittle.

The zirconium increases the strength of the niobium base alloy, and at least 0.5% zirconium is desirable to maintain this property at a sufiiciently high level. If zirconium is present in a quantity greater than about 2%, the niobium base alloy becomes susceptible to atmospheric contamination.

The above-described niobium base alloy has been prepared by alloying high purity elemental raw materials in an inert atmosphere of argon plus helium. It was found that the constituents of the alloy may be added either simultaneously or successively. Melting was accomplished with a nonconsumable tungsten electrode electric arc. After solidification of the alloy, it was crushed to fine particle size and re-melted in the above-described manner and cast in ingot form. This ingot was enclosed in a container formed of an alloy of molybdenum plus 0.5% titanium, heated to approximately 2700" F., and extruded. Sound extrusions may be obtained with the niobium base alloy of this invention by direct extrusion of the cast ingot, the extruded alloy having a fibrous appearing cold worked structure. Subsequent further working has been accomplished by both swaging and rolling.

Stress-rupture test bars were machined from this extruded niobium base alloy bar stock and tested in an argon atmosphere at a temperature of 2000 F. under progressively increasing loads from 10,000 p.s.i. to 25,000 p.s.i. For example, an alloy consisting of about 82% niobium, 10% molybdenum, 4% chromium, 2% titanium and 2% zirconium had a hot strength estimated to be appreciably in excess of 25,000 p.s.i. for more than hours at a temperature of 2000" F. The results of this particular test were as follows.

Stress-Rupture Test at 2000" F. in Argon Atmosphere Stress in p.s.i.: Hours prior to next higher stress 10,000 1 20 15,000 100 20,000 20 25,000 102 (failed) Total elongation, measured on the machined test bar halves after rupture, was approximately 47%. A reduction in area of 54.6% was measured in the test bar gauge length. These data show that the niobium base alloy possesses the combination of excellent oxidation resistance at extreme temperatures and high hot strength. The alloy has a melting point materially in excess of 3000 F the desired minimum hereinbefore mentioned.

While my invention has been described by means of certain specific examples, it is to be understood that the scope of the invention is not to be limited thereby except as defined by the following claims.

I claim:

1. A niobium base alloy characterized by good hot strength and outstanding oxidation resistance at elevated temperatures, said alloy consisting essentially of about 2% to 5% titanium, 9% to 11% molybdenum, 4% to 8% chromium, 0.5% to 2% zirconium and the balance substantially all niobium.

2. A gas turbine blade characterized by outstanding oxidation resistance upon exposure to oxidizing gases at a temperature of 2000 B, said blade being formed of an alloy comprising about 2% to 5% titanium, 9% to 11% molybdenum, 4% to 8% chromium, 0.5% to 2% zirconium and 76% to 84% niobium.

3. A high temperature niobium base alloy having an outstanding combination of hot strength, fabricability, oxidation resistance and a stress-rupture life of more than 100 hours under a load of 15,000 p.s.i. at a temperature of 2000 F., said alloy consisting essentially of about 2% 4 to 5% titanium, 9% to 11% molybdenum, 4% to 8% chromium, approximately 2% zirconium and 76% to 84% niobium.

4. An oxidation-resistant high temperature niobium base alloy consisting essentially of about 2% titanium, 10% molybdenum, 4% chromium, 2% zirconium and the balance niobium.

Rhodin Apr. 14, 1959 Wainer Apr. 21, 1959 

2. A GAS TURBINE BLADE CHARACTERIZED BY OUTSTANDING OXIDATION RESISTANCE UPON EXPOSURE TO OXIDIZING GASES AT A TEMPERATURE OF 2000* F., SAID BLADE BEING FORMED OF AN ALLOY COMPRISING ABOUT 2% TO 5% TITANIUM, 9% TO 11% MOLYBDENUM, 4% TO 8% CHRONIUM, 0.5% TO 5% ZIRCONIUM AND 79% TO 84% NIOBIUM. 